I worked in a lab at Univserity that had several projects using gait analysis. It is very identifying, but much harder to collect than this kind of data -- you need a physical device on the target.

Still, it could be very useful for intelligence agencies. If they have identified that a phone is being used by a terrorist they can identify which terrorist is carrying it if they're able to get access to its accelerometer. Or they could gain access to millions of phones' accelerometers and identify which ones are being used by known terrorists. At the very least I expect to see it in some spy movies in the next few years.

Given the recent leaks, I think we should start using "terrorist" in quotes, at least when referring to NSA "terrorists".

Don't most people carry a mobile with them?

Yes, but I don't think the phone companies save your accelerometer data like they save your phone call metadata. You'd have to get access to the phone's accelerometer somehow.